A wind power plant (WPP) is an energy converting system consisting of one or more wind turbine generator(s), which converts kinetic wind energy into electrical energy for utility power grids. Specifically, wind is applied to wind turbine blades of the wind turbine to rotate a rotor. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. The electrical energy from the wind turbine generator(s) is fed into an electrical grid at a point of common coupling (PCC).
It has been shown that a problem exists in the case where a wind power park experiences a LVRT (low voltage ride through) or other failure modes in the electrical grid. Whenever a wind power plant experiences a LVRT—due to a problem on the grid—the voltage V is reduced, in some cases down to zero for a while (the order 100 milliseconds), where after the voltage V and therefore also output power P is ramped up.
Today, when the wind power plant is controlled by voltage control, the wind power plant checks that the voltage is at the required value—e.g. 95% of the maximum voltage—and if affirmative, the wind power plant is within the normal voltage level of the grid. Even though the voltage level V is at the intended value and from the wind power plant side of view, everything is fine; the grid may not be strong enough to support the amount of electrical power. This may induce a new LVRT from which the WPP must recover. Thus, at voltage control of at wind power plant, a LVRT occurring as a fault in the grid, may result in the wind power plant induces a number of LVRT and subsequently disconnects from the grid. The normal solution to this problem is to strengthen the grid. This solution is of course expensive and time-consuming. Another consequence of this is often that the amount of installed capacity of wind power is lower than the actual potential of the wind site. In some cases the wind turbine generators are operated in reduced power mode, as described in EP 1222389.
Electrical grid operators want to run a stable grid, meaning they don't want to come near the stability margins of the grid.
EP 1222389 relates to a method for operating a wind farm that consists of at least two wind turbines, the power provided by these wind energy turbines being limited in amount to a maximum possible input to the network which is less than the maximum possible value of power to be provided (nominal power). The maximum possible value to be input is determined by the receiving capacity (power capacity) of the network into which the energy is input and/or by the power capacity of the energy transmission unit or the transformer by which means the energy generated by the wind turbine is input into the network.
In order to increase the amount of wind power fed into the electrical grid, measurements of the actual short circuit impedance of the grid can be made and voltage control is done in respect to the gain value of electrical grid.
This is the content of patent application WO 2009/083448 that relates to a method for controlling a voltage level of a power supply grid operationally connected to a source of electrical power, the method comprising the steps of determining a short circuit impedance of the power supply grid at a point of common coupling, calculating, using the determined short circuit impedance, a gain value of the power supply grid, and controlling the grid voltage level in accordance with the calculated gain value by applying said gain value as a gain parameter in a voltage controller.
Both documents help to solve the problem of operating the grid in a stable mode, but it might not be a solution that allows the largest penetration of electrical power into the grid and it might not be the fastest way to overcome an LVRT event.
It is thus an object of the invention to provide an improved method of maintaining stable grid, while the recovery after the LVRT is done in a fast way, and while being able to feed in as much active power as the stability of the electrical grid allows.